There are many applications in industry and commerce where it is desirable, and in some cases required, to suppress fluid-borne noise in hydraulic power systems and other fluid handling systems. As an example, it is desirable to attenuate or suppress fluid-borne noise generated by the pump or fluid valving in power steering, power brake, fuel distribution and air conditioning systems.
The inherent design of fluid pumps, whether driven by an internal combustion engine, an electric motor or by fluid system valves, causes pressure fluctuations or pulses in the fluid line which generate fluid-borne noise. The pistons, rotors, gears, vanes or other fluid displacement elements that pump the fluid cause pressure fluctuations, ripple, or pulses within the fluid at a frequency that is dependent upon pump speed. The geometry and inherent characteristic of the pump can also be sources of fluid pressure fluctuations and vibrations. This fluid ripple can be a source of audible and objectionable noise and can also excite components along its path (e.g., the steering gear in power steering) to cause them to become secondary generators of such noise.
During normal operation of an automotive power steering system, for example, hydraulic fluid pressure can repetitively vary and thereby generate a pressure dependent wave form that can range substantially in magnitude or amplitude between the upper and lower limit values and induce system vibration. The frequency of such fluid-borne vibration also can vary substantially with the speed of the driving component (e.g., engine) and other factors. Therefore expansible-type hoses are often used as the fluid conductors in fluid systems in order to dampen and absorb such fluid-borne vibrations. These hoses typically consist of a tube of rubber or another elastomeric material which is reinforced by nylon or a similar material. The braiding or other reinforced member may be disposed within the outer circumference of the tubing, or may be disposed within a layer of elastomeric material that is itself disposed around the outside of the tubing. The soft, compressible, elastic material of expansible hose expands upon pressure to absorb pressure fluctuations in the fluid. The strengthening braid also allows some degree of expansion when subjected to pressure.
Expansible hoses are wide-band devices and, in principle, can respond to fluid vibrations over a wide frequency range. For satisfactory performance, there must be enough expansion capability in the elastomeric hose material to absorb the pressure fluctuations over the amplitude and frequency range encountered in the fluid system. However, this is possible only when the changes in volume flow rate associated with the pressure ripples are less than the volume expansion capability of the hose for the same change in hydraulic fluid pressure.
In order to dampen the fluctuation even further, an attenuator in the form of a tuner conduit made of spirally constructed steel or smooth wall polymeric material, such as Teflon or nylon, also has been used within the hose. This tuner usually permits the fluid to flow, via clearances between the spiral tube or apertures in the tube, from within its bore into the annulus or chamber formed between the tuner O.D. and the hose I.D. or bore. The fluid flowing in this annulus meets the fluid which is flowing inside the tuner bore at the downstream end of the tuner length and results in some reduction of the pressure pulsation and resultant noise and/or vibration.